This invention relates to a hydroxyalkanoate polymer composition, and more specifically, to a resin composition comprising a hydroxybutyrate homopolymer or a hydroxybutyrate-hydroxyvalerate copolymer. This invention also relates to a device fabricated from such a resin composition.
Hydroxybutyrate homopolymer and a hydroxybutyrate-hydroxyvalerate copolymer are formed in a bacteria or other microorganisms for their energy source, and these polymers are biodegradable thermoplastic polyesters which are decomposed and digested by various microorganisms commonly present in soil. It is also known that these polymers are degradable in an organism without stimulating any hazardous reaction. For example, N. D. Miller and D. F. Williams disclose that hydroxybutyrate homopolymer and a hydroxybutyrate-hydroxyvalerate copolymer are decomposed in an organism through a hydrolyric process and an enzymatic process. See Biomaterial, vol. 8, page 129, 1987. T. Saito et al. also disclose similar results. See Biomaterial, vol. 12, page 309, 1991. There is also reported in J. Thorac. Cardiovasc. Surg., vol. 104, page 600, 1992 that use of a hydroxybutyrate homopolymer for an artificial heart membrane resulted in a favorable tissue response.
Hydroxybutyrate homopolymer was discovered several decades ago. However, it was not put into a practical use until quite recently since its extraction from the bacteria and subsequent purification were quite difficult, and since its decomposition point (183.degree. C.) and melting point (180.degree. C.) were too close to enable a uniform molding. Backed by recent heightening in the interest in environmental issues, there were developed various techniques for copolymerizing the hydroxybutyrate with another hydroxyalkanoate (Japanese Patent Application Kokai (unexamined laid open) No. 57(1982)-150393 and 59(1984)-220192) as well as purification processes (Japanese Patent Application Kokai Nos. 57(1982)-174094 and 60(1980)-145097) to enable supply of lower melting materials with a relatively improved moldability. Hydroxybutyrate homopolymer inherently suffers from a lower crystallization speed compared to other commonly used synthetic materials, and in particular, hydroxybutyrate-hydroxyvalerate copolymers suffer from a decrease in crystallization speed with an increase in the hydroxyvalerate content. In the case of injection molding, an excessively low crystallization speed would lead to a difficulty in temperature control upon solidification of the resin in the mold to result in molding failures due to an insufficient solidification or "sinkmarks", and an unduly prolonged molding cycle would lead to an increased production cost. In the case of extrusion molding, insufficiently crystallized articles would adhere to each other even at room temperature and consequently, such molded articles would be quite inconvenient to handle. Furthermore, low crystallization speed is likely to induce a growth of large spherulites to result in brittle molded articles.
In view of such situation, a crystallization nucleating agent is generally incorporated in such a resin to thereby improve precision of the molded articles upon injection molding or extrusion, to provide a mirror surface with the molded articles, and to improve various other properties of the molded articles. For example, there is described in Japanese Patent Publication (examined laid open) No. 52(1977)-5935 that an addition of dibenzalsorbit to polypropylene as a nucleating agent upon melting and molding of the resin is effective in decreasing shrinkage upon molding and in preventing "sinkmarks" of the molded articles. There is also described in Japanese Patent Application Kokai No. 3(1991)-24151 that an addition of a metal compound in combination with organic sulfonic acid or organic phosphinic acid to a hydroxyalkanoate polymer would result in a high crystallization and a decreased brittleness of the molded articles.
Boron nitride is generally known to be a nucleating agent for resins, and boron nitride proved to be a good nucleating agent for the hydroxyalkanoate polymer of the present invention. However, boron nitride is less likely to undergo degradation under natural environment, and consequently, the boron nitride used as the nucleating agent is likely to remain undecomposed even after the degradation of the resin composition. In particular, when boron nitride is incorporated into various devices to be implanted in a living body, which is designed to be decomposed and absorbed in the living body, the boron nitride remained unabsorbed may generate a chronic inflammation or a cancer. The process of Japanese Patent Application Kokai No. 3(1991)-24151, wherein a polyhydroxyalkanoate is admixed with a combination of a metal compound and organic phosphonic acid or organic phosphinic acid, also requires an adequate selection of the materials which are nontoxic to a living body. Such a selection should require considerable experimentation since a combination of two compounds should be evaluated for their toxicity and nucleating effects.